A conventional thermal fixing device includes an endless fixing (fusing) belt, a nip plate that slidingly contacts an inner peripheral surface of the fixing belt, a backup roller that is pressed toward the nip plate via the fixing belt, a guide member that supports the nip plate, and a stay that supports the guide member. In this fixing device, the stay is disposed at a side opposite to the back-up roller with respect to the nip plate such that the stay receives, via the guide member, a pressing force applied from the back-up member to the nip plate.